// ----rawudp.c------
// Must be run by root lol! Just datagram, no payload/data
#include <unistd.h>
#include <stdio.h>
#include <sys/socket.h>
#include <netinet/ip.h>
#include <netinet/udp.h>

// The packet length
#define PCKT_LEN 8192

// Can create separate header file (.h) for all headers' structure
// The IP header's structure
struct ipheader {
	unsigned char      iph_ihl:5, iph_ver:4;
	unsigned char      iph_tos;
	unsigned short int iph_len;
	unsigned short int iph_ident;
	unsigned char      iph_flag;
	unsigned short int iph_offset;
	unsigned char      iph_ttl;
	unsigned char      iph_protocol;
	unsigned short int iph_chksum;
	unsigned int       iph_sourceip;
	unsigned int       iph_destip;
};

// UDP header's structure
struct udpheader {
	unsigned short int udph_srcport;
	unsigned short int udph_destport;
	unsigned short int udph_len;
	unsigned short int udph_chksum;
};
// total udp header length: 8 bytes (=64 bits)

// Function for checksum calculation. From the RFC,
// the checksum algorithm is:
//  "The checksum field is the 16 bit one's complement of the one's
//  complement sum of all 16 bit words in the header.  For purposes of
//  computing the checksum, the value of the checksum field is zero."
unsigned short csum(unsigned short *buf, int nwords)
{       //
	unsigned long sum;
	for(sum=0; nwords>0; nwords--)
		sum += *buf++;
	sum = (sum >> 16) + (sum &0xffff);
	sum += (sum >> 16);
	return (unsigned short)(~sum);
}
// Source IP, source port, target IP, target port from the command line arguments
int main(int argc, char *argv[])
{
	int sd;
	// No data/payload just datagram
	char buffer[PCKT_LEN];
	// Our own headers' structures
	struct ipheader *ip = (struct ipheader *) buffer;
	struct udpheader *udp = (struct udpheader *) (buffer + sizeof(struct ipheader));
	// Source and destination addresses: IP and port
	struct sockaddr_in sin, din;
	int one = 1;
	const int *val = &one;

	memset(buffer, 0, PCKT_LEN);

	if(argc != 5)
	{
		printf("- Invalid parameters!!!\n");
		printf("- Usage %s <source hostname/IP> <source port> <target hostname/IP> <target port>\n", argv[0]);
		exit(-1);
	}

	// Create a raw socket with UDP protocol
	sd = socket(PF_INET, SOCK_RAW, IPPROTO_UDP);
	if(sd < 0)
	{
		perror("socket() error");
		// If something wrong just exit
		exit(-1);
	}
	else
		printf("socket() - Using SOCK_RAW socket and UDP protocol is OK.\n");

	// The source is redundant, may be used later if needed
	// The address family
	sin.sin_family = AF_INET;
	din.sin_family = AF_INET;
	// Port numbers
	sin.sin_port = htons(atoi(argv[2]));
	din.sin_port = htons(atoi(argv[4]));
	// IP addresses
	sin.sin_addr.s_addr = inet_addr(argv[1]);
	din.sin_addr.s_addr = inet_addr(argv[3]);

	// Fabricate the IP header or we can use the
	// standard header structures but assign our own values.
	ip->iph_ihl = 5;
	ip->iph_ver = 4;
	ip->iph_tos = 16; // Low delay
	ip->iph_len = sizeof(struct ipheader) + sizeof(struct udpheader) + 1;
	ip->iph_ident = htons(54321);
	ip->iph_ttl = 64; // hops
	ip->iph_protocol = 17; // UDP
	// Source IP address, can use spoofed address here!!!
	ip->iph_sourceip = inet_addr(argv[1]);
	// The destination IP address
	ip->iph_destip = inet_addr(argv[3]);

	// Fabricate the UDP header. Source port number, redundant
	udp->udph_srcport = htons(atoi(argv[2]));
	// Destination port number
	udp->udph_destport = htons(atoi(argv[4]));
	udp->udph_len = htons(sizeof(struct udpheader));
	// Calculate the checksum for integrity
	ip->iph_chksum = csum((unsigned short *)buffer, sizeof(struct ipheader) + sizeof(struct udpheader));
	// Inform the kernel do not fill up the packet structure. we will build our own...
	if(setsockopt(sd, IPPROTO_IP, IP_HDRINCL, val, sizeof(one)) < 0)
	{
		perror("setsockopt() error");
		exit(-1);
	}
	else
		printf("setsockopt() is OK.\n");

	// Send loop, send for every 2 second for 100 count
	printf("Trying...\n");
	printf("Using raw socket and UDP protocol\n");
	printf("Using Source IP: %s port: %u, Target IP: %s port: %u.\n", argv[1], atoi(argv[2]), argv[3], atoi(argv[4]));

	int count;
	for(count = 1; count <=20; count++)
	{
		if(sendto(sd, buffer, ip->iph_len, 0, (struct sockaddr *)&sin, sizeof(sin)) < 0)
			// Verify
		{
			perror("sendto() error");
			exit(-1);
		}
		else
		{
			printf("Count #%u - sendto() is OK.\n", count);
			sleep(2);
		}
	}
	close(sd);
	return 0;
}
